I just want to show the difference between IP.Board and Simple Machines and made therefore some screen casts.
I think, You can easliy see, why www.Poultrychat.com and www.urban-chickens.com use IP.Board.

The graphical surface has a higher quality in IP-Board. In the messenger You can edit messages until the recipient read it.
In Simple Machines the surface (e.g. the smilies) look cheaper (blurred). In the messenger it is not possible to edit the messages at any time!
For me still rules IP.Board ... even if Simple Machines is for free!